Because of excellent compatibility with human bone, artificial bone made of apatite can be bonded to the human bone directly. Accordingly, the artificial bone made of apatite has recently been appreciated for effectiveness, finding clinical applications in cosmetic surgery, neurosurgery, plastic surgery, oral surgery, etc. However, artificial ceramic bone such as apatite is not necessarily completely identical with human bone in terms of mechanical properties and physiological properties. For instance, a so-called artificial ceramic bone made only of apatite is harder and more brittle than the human bone. While the human bone is repeatedly subjected to metabolism of absorption and regeneration, the artificial bone made of apatite is not substantially dissolved but semi-permanently remains in the human body. The remaining artificial bone breaks the human bone at their interface, making it likely to cause bone fracture.
Research has recently become active on artificial bone decomposable in the human body, which is closer in composition to human bone than the artificial apatite bone, and various proposals have been made. For instance, JP 11-513590 A discloses a porous body having a network structure, in which collagen and, if necessary, other binders are bonded to hydroxyapatite. Human bone is formed in the porous body, and the porous body is biologically decomposed and absorbed by the human body. Accordingly, this porous body can be used for the fixation of vertebra, the filling of bone defects, the repair of fractured bone, the grafting of periodontal defects, etc. However, this porous body is a mere mixture of collagen and apatite, which does not have a similar structure to the living bone, namely a structure in which the C-axis of apatite is oriented along collagen fibers. Further, it does not have sufficient mechanical strength, and is poor in bone-forming capability.
To solve this problem, the inventors previously disclosed a porous body that can be absorbed by the body according to a similar mechanism to the living bone, and has high bone-forming capability (WO 2004/041320 A1). This porous body is obtained by freeze-drying a dispersion comprising an apatite/collagen composite and collagen to form a porous body, and cross-linking collagen in the porous body. When the apatite/collagen composite contained in the porous body embedded in a living body is dissolved and/or decomposed, it is considered that there appears an environment suitable for forming bone in and around the porous body. The dissolved and/or decomposed apatite/collagen composite is absorbed by a newly formed living bone. However, further research by the inventors has revealed that the apatite/collagen composite is covered with collagen, a binder, in this method. As a result, some part of the apatite/collagen composite does not appear on the porous body surface, so that the porous body is not well absorbed in the course of regeneration of the living bone.